Not applicable.
1. The Field of the Invention
The present invention generally relates to high voltage devices. More particularly, the present invention relates to an apparatus and method for adjusting voltage potentials on the surface of insulating structures used in high voltage devices.
2. The Relevant Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode generally comprises a metallic cathode head housing a filament that, when heated via an electrical current, emits electrons. The cathode is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode. The anode, which typically comprises a graphite substrate upon which is disposed a heavy metallic target surface, can be stationary within the vacuum enclosure, or can be rotatably supported by a rotor shaft and a rotor assembly. The rotary anode is typically spun using a stator. Often, the vacuum enclosure is disposed within an outer housing for cooling and insulating purposes.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. A high electric potential, or voltage, placed between the cathode and anode causes the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. The point at which the electrons strike the target surface is referred to as the focal spot. Upon approaching and striking the focal spot, many of the electrons convert their kinetic energy and either emit, or cause the target surface material to emit, electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (xe2x80x9cZ numbersxe2x80x9d), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The target surface of the anode is angled to minimize the size of the resultant x-ray beam, while maintaining a sufficiently sized focal spot. The x-ray beam is collimated before exiting the x-ray tube through windows defined in the vacuum enclosure and outer housing. The x-ray beam is then directed to the x-ray subject to be analyzed, such as a medical patient or a material sample.
Several types of x-ray tubes are commonly known in the art. Double-ended x-ray tubes electrically bias both the cathode and the anode with a high negative and high positive voltage, respectively. The voltage applied to the cathode and anode may reach +/xe2x88x9275 kilovolts (xe2x80x9ckVxe2x80x9d) or higher during tube operation, depending on the type of x-ray tube. In contrast, single-ended x-ray tubes electrically bias only the cathode, while maintaining the anode at the housing or ground potential. In such tubes, the cathode may be biased with a voltage of xe2x88x92150 kV or more during tube operation. In either case, a sufficient differential voltage is established between the anode and the cathode to enable electrons produced by the cathode filament to accelerate toward the target surface of the anode.
Because of the high voltage differential present between them, an electric field is created between the anode and the cathode during tube operation. The high voltages present at the anode and/or cathode also necessitate the use of insulating structures supportably connecting the anode and/or cathode to the vacuum enclosure or outer housing to electrically isolate them from the rest of the tube. These insulating structures are typically composed of an insulative material, such as glass or ceramic, and may comprise a variety of shapes. Regardless of their shape however, the insulating structures must accommodate the reduction in voltage from the high voltage present at the anode and/or cathode to the much lower housing or ground potential typically present at the surface of the vacuum enclosure.
The interaction of the electric field with the insulating structures for the anode and/or cathode creates a voltage potential distribution along the insulating length of the insulating structure. The insulating length is defined as the length of insulating structure existing between the high voltage source and the low voltage device structure. In an x-ray tube, the insulating length of the insulating structure extends from the anode and/or cathode to the vacuum enclosure, with high voltage present in the insulating structure near the anode or cathode, and low voltage in the insulating structure near the enclosure. In this way, the high voltage of the electric field is gradually dissipated along the length of the insulating structure, thereby electrically isolating the anode and/or cathode and protecting other tube components.
It has been discovered that during tube operation, the voltage potential distribution in the insulating structures created by the electric field existing between the anode and the cathode tends to concentrate near the high voltage source, in this case the anode and/or cathode. Among other things, this field concentration causes the overall voltage drop between the high voltage source and the vacuum enclosure to occur over a shorter distance of the insulating structure than the entire length thereof In other words, a portion of length of the insulating structure is not utilized to accommodate the necessary voltage drop between the anode and/or cathode and the enclosure. Several problems are created by this field concentration in the insulating structure. First, a waste of insulating structure occurs because a portion of the structure nearest the vacuum enclosure is not utilized. Worse, however, is an added per unit electric field stress that is imposed on the portion of the insulating structure nearest the anode and/or cathode, where the field concentration occurs. This electric field stress is highly undesirable because it may weaken over time the structural integrity of the x-ray tube. Eventually, the insulating structure may fail, causing substantial damage to the x-ray tube and requiring much time and expense to correct.
Various solutions have been attempted to resolve the effects caused by the electric field concentration near the anode and/or cathode. One attempted solution has involved increasing the size of the insulating structure near the anode and/or cathode in order to spread out the electric field concentration, and thus the electric field stress. Such a solution may be undesirable or impossible, however, given the tight space constraints present in many high voltage devices, especially x-ray tubes.
A need therefore exists to provide a manner by which electric field stress present in insulating structures of high voltage devices, such as x-ray tubes, may be mitigated. More generally, a need exists to enable the shaping of high voltage gradients along the length of an insulating structure in a high voltage device as may be desired by the operators of such devices.
In accordance with the invention as embodied and broadly described herein, the foregoing needs are met by a method and apparatus for modifying the voltage potential distribution in insulating structures, or insulators, employed in high voltage devices. Preferred embodiments of the present invention are directed to altering the boundary conditions of the surfaces of insulating structures within x-ray tubes such that the voltage potential distribution along the length of the insulators extending from the anode and/or the cathode to the vacuum enclosure is shaped as may be desired for the particular application in which the tube is employed. The present invention may also be advantageously employed in a variety of other high voltage devices where shaping of the high voltage potential distributions along insulating structures disposed therein is needed or desired.
In a first embodiment, the voltage potential distribution is modified via a coating material non-uniformly applied to the surface of the anode and/or cathode insulator within an x-ray tube. The coating material has an electrical conductivity greater than that of the surface of the insulator. In addition, the coating material is non-uniformly applied in order to adjust the voltage distribution along length of the insulator from the anode or cathode to the vacuum enclosure surface. For instance, the thickness of the coating may be more thickly applied to the surface of the insulator nearest the cathode or anode than it is applied to than the portion nearest the vacuum enclosure surface. Or, the composition of the coating material may be altered such that it possesses greater conductivity where it is applied to the insulator surface nearest the cathode or anode. In this way, the desired voltage potential distribution gradient is achieved along the length of the insulator during operation of the x-ray tube.
In a second embodiment, the surface of an insulator is modified by preferential reduction of existing material (bulk or trace) using, for example, heating in a hydrogen atmosphere; electron (or ion) beam bombardment; or chemical means. For example, the surface of an anode insulator comprising leaded glass can be modified in order to change its conductivity. In one embodiment, this is accomplished by masking portions of the inner surface of the insulator, typically comprising a funnel or cone shape. The anode insulator is then heated in a furnace having a hydrogen-rich atmosphere, thereby causing a chemical reduction of lead oxide near the insulator surface. This reduction of lead oxide increases the amount of metallic lead near the surface of the insulator, which in turn increases the conductivity of the surface. This process is repeated for different regions of the insulator as desired in order to shape the overall conductivity of the insulator surface. As with the first embodiment, this enhances the ability of the insulator surface to more evenly distribute the voltage potential along the length thereof during tube operation. Similarly, sodium or potassium could be reduced from alumino-ortho-silicate glasses. In other examples, Boron or sodium could be reduced from xe2x80x9cPyrexxe2x80x9d glass, or calcium, strontium and other metallic oxides could be reduced from the glassy phase of ceramic materials or from oxide glasses. Preferential reduction of the bulk ceramic material (such as reducing aluminum to aluminum, or silicon from silica ceramics) could also be accomplished by similar means.
It will be appreciated that the insulator surface conductivity can be modified by other means, such as preferential reduction as required. Deposition of a metallic overcoating on the insulator surface, and subsequent preferential oxidation of the metallic overcoat could also achieve the desired surface conductivity. The conductivity of insulating materials may also be modified by preferential ionic transport through the insulating material through the use of electric fields in conjunction with heating. Similar methods may also be used for grading of properties of the insulator.
In a third embodiment, an insulating structure having a smooth, continuously connecting surface is coated on at least a portion of its continuous surface with a conductive coating material similar to the material employed in the first embodiment. The coated surface is then scribed via a laser or the like to form a groove on the coated surface extending down to the surface of the insulator. This creates a conductive path along the surface of the insulator having a defined voltage gradient as characterized by the shape and path of the scribed groove. In this way, the voltage potential along the insulating length of the insulator surface is more evenly distributed.
In a fourth embodiment, the insulating structure comprises a plurality of material segments that have been joined together to form the insulator. The segments are preferably assembled by sintering and furnace heating, then shaped into the final insulator form. Each insulator segment preferably possesses a distinct electrical conductivity so that, when assembled, the insulator defines a non-uniform surface conductivity that modifies and more evenly distributes the voltage potential distribution along the insulator surface during operation of the high voltage device.
The above embodiments of the present invention enable the voltage potential distribution to be modified along the insulating length by adjusting the surface conditions of the insulator, namely, the conductivity thereof. In so doing, the problems associated with field concentration near the high voltage source may be avoided by adjusting the conductivity of the insulator such that the voltage distribution is spread more evenly along the insulator length. This, in turn, avoids complications with electric field stress arising from the concentration of the electric field near the high voltage structure. This benefit is especially useful for x-ray tubes, where the effects of the electric field stress may eventually cause catastrophic failure of the insulator and the entire tube as well.
These and other objects and features of the present invention will become more fully parent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.